Which Setup Is More Powerfull?

Two 550 Ti is more powerful but that will consume lot higher power consumption and they also have scaling issues in couple of games. By the way, both of 'em are previous gen GPU and should be avoided. Better opt for AMD HD 7000 series or Nvidia 600 series. Tell us your budget for the card and we'll suggest which card is best suited for your needs.

I Already Have A GTX550Ti And Was Confused About Going For SLI or A New GPU.
About Power Consumption, I Am Also Planning On Buying A New SMPS, I Was Thinking About Corsair TX850V2.

Why not sell it on erodov or somewhere else?